Electron microscopic analysis of RNA transcription in preimplantation rabbit embryos.

We have examined directly RNA synthesis in several stages of preimplantation rabbit embryos by electron microscopic analysis of chromatin spreads. DNA/chromatin packing ratios were determined for non-transcribing regions (2.0) and for regions active in RNA synthesis (1.1). Significant amounts of RNA transcription were observed in all embryonic stages examined. Lengths of transcribed regions were calculated and range from 1.1 to 10.9 Kb of B-form DNA. The frequency of chromatin-associated ribonucleoprotein (RNP) fibrils was greatly reduced in samples prepared from embryos treated with alpha-amanitin indicating that the RNPs observed were growing chains. The frequency of single and multiple RNP fibrils attached to chromatin is the same at all stages examined implying the absence of extensive modulation of RNA chain initiation. These results are discussed in relation to the biochemical data on RNA synthesis in preimplantation rabbit embryos.